Despite the apparent satisfaction of industry with the commercial design methods available to them, an outstanding question is the choice of the better cooling curve, differential or integral, for use with the equilibrium design approach. This is considered in the light of experimental data taken with a vapour mixture, condensing in a horizontal shell-side condenser of semi-industrial scale, (diameter 0.336 m, and length 2.438 m). This configuration is frequently used in the petroleum industry but such condensers may perform poorly because of separation of the condensate from the vapour flow, leading to a fractionation effect. Thus the vapour may become richer in the more volatile components with a fall in saturation temperature and driving force. Therefore, the area required for a given degree of condensation might be larger compared with a condensation process following the integral cooling curve. Temperature profiles, which are predicted by the integral and differential cooling curves, are compared with experiment for condensation of methanol-water mixtures of varying composition.